travis: Fix build errors due to change to non-system python
Created by: SunilMohanAdapa
When virtualenv creates an environment with --python=python3, it was earlier picking Python 3 from system at /usr/bin/python3. Due to recent changes in Travis, now it picks up Python 3.6.1 from /opt. This new Python does not take into account system packages which are being installed via apt.
Signed-off-by: Sunil Mohan Adapa sunil@medhas.org